In order to control the flow of molten steel jetted out from the immersion nozzle, it was usual to contrive the shape of the discharge port in the immersion nozzle, or to decrease the pouring rate of molten steel.
However, it was difficult to completely prevent quality defects resulting from inclusion or the like in molten steel by simply changing the shape of the discharge port or decreasing the pouring rate of molten steel.
As a prior art relating to this point, JP-A-57-17356 discloses a method of applying a braking force to the flow of molten steel jetted out from the immersion nozzle by arranging a device for generating a static field in the mold for the continuous casting, and JP-A-2-284750 discloses a technique of applying a braking force to the flow of molten steel jetted out from the immersion nozzle by Lorenz force produced through an interaction between current and magnetic field induced by applying a static field to the entire mold for the continuous casting.
In the technique disclosed in JP-A-57-17356, when the braking force is applied to the jet of molten steel, the flowing direction is changed to disperse the energy inherent to the jet of molten steel as if the jet of molten steel collides with a wall and hence the uniform flow can not be obtained. Further, the jet of molten steel escapes in a direction having no static field, so that the satisfactory result can not be obtained.
In the technique disclosed in JP-A-2-284750, it is possible to attain the uniformization of molten steel jetted out from the immersion nozzle and the variation of molten steel surface on the meniscus portion can be made small, so that the surface and internal qualities of the cast slab can be improved to a certain extent, respectively. However, when the high-speed casting is carried out under a condition that the throughput of molten steel exceeds 2 times of the conventional throughput, there still remaining the following problems.
1) When using a multihole type immersion nozzle, the occurrence of deflected flow in a mold accompanied with the flow of molten steel jetted out from the immersion nozzle can not be avoided. PA1 2) In case of the multihole type immersion nozzle, when the clogging of the nozzle is caused with the increase of the flow rate of molten steel jetted, the deflected flow in the mold becomes large and hence the continuous casting can not stably be attained. PA1 3) In case of the multihole type immersion nozzle, the contrarotating flow at a short side of the mold becomes high speed accompanied with the increase of the flow rate of molten steel jetted, so that the variation of molten steel surface becomes large and the entrapment of powder can not be avoided. Moreover, the use of single-hole type immersion nozzle can be considered. In the latter case, when the static field is applied to a lower zone of molten steel jetted, the contrarotating upstream of molten steel is generated through an influence of reflection current (induction current flowing in a direction of promoting the jet of molten steel) in the mold to cause the variation of molten steel surface and hence powder is entrapped. PA1 4) Since the disorder of molten steel becomes large during the oscillation of the mold, the depth of oscillation mark becomes deeper and also the oscillation mark becomes disordered, so that surface defects (coil defect) are frequently created in the resulting rolled steel sheet. PA1 5) Since the molten steel surface is rippled inside the mold to disorder the oscillation mark, it is difficult to uniformly supply powder and hence it is apt to cause restraint breakout due to the occurrence of sticking or the like. PA1 6) There is a fear of remelting solidification shell by the flow of molten steel jetted out from the immersion nozzle.
Recently, there is proposed a continuously casting method through the application of static field to a lower end portion of a mold for the continuous casting (JP-A-7-51801, JP-A-7-51802, JP-A-59-76647, JP-A-62-254955, Iron Steel Eng., May (1984), pp 41-47, JP-A-6-126399), a continuous casting method using two nozzles while applying the static field to the lower end of the mold for the continuous casting (JP-A-5-277641) and the like.
These techniques are intended for not only the continuous casting of ordinary steel but also the casting of clad steel. In these techniques, it is possible to decrease the flow rate by applying the static field to an adequate zone (zone near to solidification shell at a side of short-side wall in the mold for the continuous casting or the like) for the flow of molten steel jetted out from the immersion nozzle, so that these techniques may sufficiently be applied to the continuous casting of ordinary steel. In any case, the value of the static field is not more than 0.5 T, so that it can not be adapted to the high-speed casting at a throughput of 6-10 t/min. Therefore, it is a disadvantage that the castable quantity is very slight without generating defects in the product.
In order to increase magnetic flux density and mitigate power cost, JP-B-63-54470 discloses a technique of exchanging the conventional normal conducting electromagnet with a superconducting electromagnet.
However, when the conditions of applying the static field are bad irrespectively of the normal conducting electromagnet or the superconducting electromagnet, there are rather frequently generated defects. Particularly, when the high-speed casting is carried out by changing the throughput of molten steel from about 5 t/min usually used to more than 6 t/min, the restrictions in the operation become severer from problems such as disorder of molten steel surface, entrapment of inclusions and the like. In this technique, there is no description on magnetic field applying conditions and casting conditions required for obtaining cast slabs having no defect.
In this connection, a casting method using a superconducting electromagnet and a cuspid magnetic field is disclosed in JP-A-3-94959. According to this method, the intensity of the magnetic field is about 0.15 T at most and is fairly small as compared with the case of using the conventional electromagnet and also the application system of the magnetic field is cusp, so that it is impossible to control the variation of molten steel surface in the mold for the continuous casting questioned in the high-speed casting.
Moreover, a method of casting slabs having less defects by applying a static field having a magnetic field intensity of 0.5 T at maximum to a lower end of the mold is disclosed in JP-A-4-52057, whereby it is possible to mitigate the entrapment of bubbles and inclusions as compared with the conventional case. However, the casting conditions are the same as in the conventional technique, so that it can not cope with the high-speed casting.
Up to the present, there is no proposal for solving the above items 1)-6) in order to realize the high-throughput, high-speed casting.
It is an object of the invention to solve the aforementioned problems when the high-speed casting is carried out at a high throughput and to provide a novel method of continuously casting steel to produce carefree cast slabs suitable for DHCR process (direct hot charged rolling process) or CC-DR process (continuous casting rolling process) as well as an apparatus suitable for carrying out this method.